Evidence for a computational distinction between proximal and distal neuronal inhibition.

Most neurons have inhibitory synapses both "proximally" near the spike-initiating zone and "distally" on dendrites. Although distal inhibition is thought to be an adaptation for selective inhibition of particular dendritic branches, another important distinction exists between proximal and distal inhibition. Proximal inhibition can attenuate excitatory input absolutely so that no amount of excitation causes firing. Distal inhibition, however, inhibits relatively; any amount of it can be overcome by sufficient excitation. These properties are used as predicted in the circuit-mediating crayfish escape behavior. Many neuronal computations require relative inhibition. This could partly account for the ubiquity of distal inhibition.